(1) Field of the Invention
This invention relates generally to amplifiers and relates more specifically to feedback amplifiers having high-gain without requiring large-valued feedback resistors.
(2) Description of the Prior Art
A well-known method for achieving high gain in feedback amplifiers without requiring large-valued feedback resistors (large resistors increase circuit area and cost in silicon implementations) is to use networks such as T- or PI-networks in the feedback path. In differential circuits the T- and PI-networks do not require an additional voltage reference to connect the T- or PI-branches to as the other side of the differential circuit can be used. However, in single-ended circuits a voltage reference is required to connect the T- or PI-branches to. When a suitable low-noise voltage is not already available then generating one may require considerable additional power and, in silicon implementations, area and cost.
A common problem in analog signal processing is converting a single-ended signal to a differential signal to achieve the advantages for working with differential circuits, such as increased dynamic range and improved rejection of common-mode interferer signals such as supply noise. Various methods exist to achieve conversion to a balanced differential signal, such as driving a truly differential amplifier with an unbalanced differential signal comprised of the original single-ended signal and a low-noise voltage reference (which may require an off-chip capacitor or low noise on-chip buffer) or using an additional inverting single-ended stage to provide the non-inverted and inverted signal pairs required in a balanced differential system.
FIG. 1 prior art illustrates a simple signal processing circuit giving single-ended to differential conversion with gain 2G. It shows a common implementation of an operational-amplifier-based gain stage (A1) with input resistor of value R Ohms and feedback resistor G times larger (G.R Ohms) providing gain (−G) in conjunction with a conventional gain (−1) stage (A2) used here to achieve single conversion of a single-ended input signal of amplitude +/− Vin to a balanced differential output of amplitude +/− (2.G.Vin), i.e. the gain to each output amounts to G, so single ended to differential conversion gain amounts to 2G.
Amplifier stages (A1) and (A2) are very well-known operational-amplifier-based inverting gain stages; each having signal gain G=(−Rf/Rin) where Rf is the feedback resistor (here having value G.R Ohms for amplifier stage (A1) and R for stage (A2)) and Rin is the input resistor (here having value R Ohms for both stages (A1) and (A2). The design techniques for such amplifier stages are familiar to many engineers.
If high gain is required because the input signal is very small, such as when e.g. the circuit of FIG. 1 prior art is used as a single-ended microphone input in an audio CODEC, then the large value of G required to achieve high gain means that the feedback resistor Rf around stage (A1) must become very large (assuming that other system specifications prohibit gain increase by reducing the stage (A1) input resistance Rin to less than R to increase the ratio Rf/Rin). In silicon implementations of this circuit, the feedback resistor Rf will then require a large silicon area, which increases the final circuit area and hence die cost.
It is a challenge for the designers of amplifiers designing circuits allowing high-gain single-ended to differential conversion circuits using low-valued resistors without requiring a low-noise reference voltage to connect a T-network to.
There are known patents dealing with amplifiers using a T-network:
U.S. Patent Publication (US 2011/0050359 to Yahav et al.) discloses a signal conversion apparatus including first and second input ports and first and second output ports. A first splitter is coupled to convert a first single-ended signal received on the first input port into a differential signal including first and second opposite-phase components, and to provide the first and second opposite-phase components respectively on the first and second output ports. A second splitter is separate from the first splitter and is coupled to convert a second single-ended signal received on the second input port into a common-mode signal including first and second in-phase components, and to provide the first and second in-phase components respectively on the first and second output ports together with the first and second opposite-phase components.
U.S. Patent (U.S. Pat. No. 7,720,444 to Darabi et al.) proposes a transceiver with a receiver, a transmitter, a local oscillator (LO) generator, a controller, and a self-testing unit. All of these components can be packaged for integration into a single IC including components such as filters and inductors, and a controller for adaptive programming and calibration of the receiver, transmitter and LO generator. A self-testing unit generates test signals with different amplitudes and frequency ranges. The test signals are coupled to the receiver, transmitter, and LO generator. A receiver front end includes a low noise amplifier (LNA), which provides high gain with good noise figure performance.
U.S. Patent (U.S. Pat. No. 7,787,642 to Baker et al.) discloses a low power high dynamic range microphone amplification system. The system includes a current sensing amplifier for receiving an input current signal representative of auditory information and for providing an amplifier output signal. The current sensing amplifier includes a DC bias network that includes a cascode filter. A T-network network in effect forms a current divider at high frequencies and attenuates current in the feedback path. Thus, it provides gain at high frequencies. At low frequencies, there is no current attenuation, so the DC gain is lower. The T-network in this case uses a capacitor for connecting the T-branch to. This approach is not feasible for integrated circuits processing signals with frequency content from dc to medium frequencies (such as audio signals), as very large capacitances would be needed to provide a low reactance at signal frequencies; implementing even small capacitors can require very considerable silicon area.
Furthermore the following textbook describes generally design of amplifiers:    (1) “The Art of Electronics”, Paul Horowitz and Winfield Hill, second edition, Cambridge University Press, ISBN 0-521-37095-7.